License: common: error_query: fix typo in SPDX specifier

The error-query API files were erroneously licensed under "GPL-2.1", a
license which doesn't exist.

As the author of those files, I hereby confirm that the intention was to
license these files under LGPL-2.1 as evidenced by their presence in the
libcommon_lgpl internal library.

Reported-by: Christophe Bedard <christophe.bedard@apex.ai>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I0d56372ca009e44732e0737bd8f10ad4a4d000c5